The advent of specialized, electronic components has made it possible to design and build sophisticated audio and video devices which are capable of producing high-quality reproductions of sounds, pictures and the like. One type of component often used is a digital signal processor (xe2x80x9cDSPxe2x80x9d). Generating audio or video signals requires other electronic components in addition to DSPs. One common component is a coder-decoder, otherwise known in the art as a xe2x80x9ccodecxe2x80x9d.
Typically, original audio signals (e.g., sounds) and video signals (e.g., pictures) are made up of xe2x80x9canalogxe2x80x9d signals. The function of a codec is to convert or xe2x80x9ccodexe2x80x9d an analog signal into a xe2x80x9cdigitalxe2x80x9d one so that the signal can be processed and the like. Likewise, it is sometimes desirable to convert or xe2x80x9cdecodexe2x80x9d a stored digital signal into an analog signal for output to another device such as an audio speaker or video display. The differences between analog and digital signals are well known in the art and need not be discussed here in great detail.
xe2x80x9cSingle channelxe2x80x9d codecs perform the coding/decoding just discussed on one audio/video xe2x80x9cchannelxe2x80x9d or signal. One advantage of using a DSP in combination with a codec is the ability to xe2x80x9cmultiplexxe2x80x9d or combine a number of signals into one signal. Multiplexing may be envisioned as the interleaving of data from separate signals to form one signal. It is possible, therefore, for a codec to output a multiplexed signal (or as the case may be, output a xe2x80x9cdemultiplexedxe2x80x9d signal). Codecs which use xe2x80x9ctime-multiplexingxe2x80x9d operate by designating certain time slots for each signal. Interleaving is accomplished by taking data from each signal only during predetermined time slots. Again, in general, multiplexing is well known in the art and need not be discussed here in detail.
There are, however, problems associated with time-multiplexing multiple channels using a codec/DSP combination. One problem relates to xe2x80x9csampling noisexe2x80x9d. An analog signal can be divided into two portions; a xe2x80x9csignalxe2x80x9d portion which consists of useful data and a xe2x80x9cnoisexe2x80x9d portion which consists of interfering, non-useful data. At some point, a digitized signal is converted back into an analog signal by the digital-to-analog conversion section of a codec. The analog signal must then be output from the codec to an external device, such as a speaker or display. Digital signals are typically xe2x80x9cclockedxe2x80x9d out of a digital-to-analog converter and fed to a single xe2x80x9csample and holdxe2x80x9d circuit. During the process of sampling and holding the signal, sampling noise is introduced.
Attempts have been made to reduce sampling noise by increasing the rate at which the digital signal is fed to the digital-to-analog converter and sampled/held. It is sometimes not feasible to do so, however, because in some instances the sampling rate and/or clock frequency is fixed. As a last resort, another electronic device, called a filter, is connected to the output of the codec. The filter effectively removes the noise but, it is an expensive solution.
Accordingly, it is an object of the present invention to provide devices and methods which reduce sampling noise in sample and hold circuits.
It is a further object of the present invention to provide devices and methods which reduce sampling noise in an analog signal output from a digital-to-analog converter.
It is yet another object of the present invention to provide devices and methods which reduce sampling noise in an analog signal output from a codec, such as a speech codec.
Other objectives, features and advantages of the present invention will become apparent to those skilled in the art from the following description taken in conjunction with the accompanying drawings.
The present invention provides novel devices and methods which reduce sampling noise in an analog signal without the need to increase the sampling rate or use additional external filters.
The novel devices and methods apply linear interpolation to sampled analog signals. In one illustrative embodiment, a novel device comprises two sample and hold circuits which alternatively transfer voltages derived from portions or samples of an analog, input voltage signal to an output capacitance. For each sample input, the novel device and/or method outputs two signals. One of the sample and hold circuits transfers a voltage which creates an output or xe2x80x9cinterpolatedxe2x80x9d voltage midway between the input voltage and an historical output voltage, while the second sample and hold circuit transfers a voltage sufficient to create an output voltage approximately equal to an input voltage. By generating two output voltage signals for every input voltage signal, sampling noise is reduced.